The invention is in the field of the conversion of guided modes of light waves in integrated optical devices. More in particular, the invention relates to a polarization converter for the conversion of a fraction of a signal component, propagating according to a first guided mode, of an optical signal into a signal component propagating according to a second guided mode, the first and second modes relating to different polarization modes, comprising a channel-type waveguide supported by a substrate, in which waveguide a periodic coupling takes place between the said guided modes of the optical signal propagating in the waveguide, said waveguide comprising an incoming wave-guiding section, an intermediary wave-guiding section and an output wave-guiding section, the intermediary wave-guiding section having a periodic geometrical structure including a periodic concatenation of, within a period length, two wave-guiding subsections, the lengths of the subsections and the number of periods being tailored to the desired conversion fraction.
The invention further relates to a method for fabricating a channel-type waveguide which is supported by a substrate and comprises a concatenation of wave-guiding sections having alternately a first and a second mutually different waveguide profile, each defined by a strip-type structure, which channel-type waveguide can be used as an intermediary waveguide in a passive polarization converter of the abovementioned type.
Reference [1] (see under References) discloses such passive converters, not only for conversions between guided modes having different polarizations, but also between guided modes having the same polarization. The conversion effect is based on symmetry of the coupling planes between the different successive wave-guiding sections, which are situated, by a suitable choice of the lengths of the subsections, at such mutual distances that positive interference is possible between the subfractions, converted in each coupling plane, of a specific guided mode. For an integrated version of such a mode converter, the symmetry required of the coupling planes is accomplished, for example, by differences in width of the different successive sections, or by successive sections having identical profiles being joined to one another in an offset manner. Although such discontinuities in a lateral direction are relatively easy to fabricate, an example provided of a polarization converter (see Example 1, page 7, lines 37-53) shows that the conversion efficiencies in the coupling planes are rather low. Consequently, in such a comparator, converting a desired fraction requires a relatively large number of coupling planes, and as a result the length of such a component is relatively large.
Further, waveguide structures are known which are provided with periodic discontinuities in waveguide profile in a vertical direction, i.e. perpendicular to the plane of the substrate, as for example from references [2] and [3]. Thus reference [2] discloses a polarization rotation mechanism on the basis of a periodic perturbation in a ridge-type waveguide by means of an asymmetric periodic coverage, disposed in the longitudinal direction on the ridge of the guide, with roof-shaped elements which per subsection are alternately located on the left-hand side and the right-hand side of the ridge. Reference [3] describes a passive polarization converter based on a periodic waveguide structure obtained by applying, over a periodically interrupted edge or a stepped recess etched into the substrate, in the longitudinal direction, the layer structure for a ridge-type waveguide. As a result, the wave-guiding subsections underneath which such an edge is situated, have a somewhat tilted waveguide profile with respect to the waveguide profile of the subsections situated in-between without such an edge. Although much higher coupling factors are obtained by using such periodic discontinuities in waveguide profile in a vertical direction, the losses as a result of attenuation are likewise much greater. Moreover, the fabrication of the periodic waveguide structures known from the references [2] and [3] is rather laborious.